The present invention relates in general to data communication systems, and in particular to method and circuitry for providing differential immunity from noise sources in data communication channels that share a reference signal among multiple data signals.
Conventional differential signaling uses a pair of wires to transmit an electrical signal. Due to its differential nature, this type of signal transmission rejects common-mode noise sources and thus enjoys higher immunity to noise and crosstalk. Differential signaling, however, doubles the amount of I/O hardware and integrated circuit (IC) pin count per signal. This overhead becomes prohibitively costly in applications where the IC processes multiple parallel data channels. Differential signaling based on the same principles but using a single reference channel that is shared by multiple single-wire data channels has been developed to reduce the required hardware overhead while retaining the noise immunity advantages. This type of differential signaling, however, is not as effective in rejecting noise at higher frequencies. Due to the fact that the reference channel is routed and connected to multiple input circuits, while a given data channel is routed and connected only to its one dedicated input circuit, inherent physical differences exist between the data channels and the share reference channel. These differences contribute to the mismatch between the parasitic capacitive loading of the data channels as compared to that of the shared reference channel. As a result, high frequency noise sources couple differently in the data channels versus the reference channel, introducing differential noise which reduces the signal to noise ratio for the communication system.
Accordingly, there is a need for method and circuitry to further improve the noise immunity of differential transmission at higher frequencies.